Reminder:
Since you are planning to use a modern solid state audio amplifier (a Symetrix 420), and since this amplifier has an output power capability that is far in excess of what is needed in this application (the amplifier is specified as capable of delivering 20 watts per channel into a pair of 4 ohm loads or 40 watts into an 8 ohm load in bridged-mono mode)... it does not matter if the load impedance that the amplifier is looking into is higher (even significantly higher) than the impedance that is specified as the "minimum" load impedance.
From the Symetrix 420 manual:
"For solid-state amplifiers, operating without any load connected or with a load whose impedance is
higher than the amplifier’s rated minimum is harmless. Of course, there is a price, and that price is
diminished power output. As the load impedance doubles, output power halves (this is a best-case
scenario and depends on actual circuit design). Thus, an amplifier having a 50-watt, 8-ohm, power
rating would only deliver 25 watts into a 16-ohm load."
With the 12k ohm 5 watt resistor across the output of the step-up transformer (as discussed in your other post:
http://amfone.net/Amforum/index.php?topic=34478.0), and with 192V (peak) across that resistor... the
peak power that the amplifier will have to deliver is 192V x 192V / 12000 ohms = 3 watts.
If you use a step up transformer with a turns ratio of 230:12 (or 115:6, depending upon whether you put the windings in series or parallel) = 19.2, the impedance that the amplifier will see is 12000 ohms / [19.2 x 19.2] = 32.7 ohms... which is perfectly fine for this application. With a 40 watt peak power rating in bridged-mono mode, into an 8 ohm load, this amplifier will deliver up to 10 watts peak power into a 32 ohm load. That's 3.3 x the power you will need to drive the grids of your modulator tubes (essentially all of that power going into the 12000 ohm resistor).
If you use a transformer with a turns ratio of 230:6 = 38.3, the impedance seen by the amplifier will be 12000 ohms / (38.3 x 38.3) = 8.2 ohms. That will also be fine.
In either case:
be careful to start with the amplifier's volume control set low so that you don't apply too large a peak voltage to the grids of the modulator tubes. In bridged mono mode, the peak output voltage of the amplifier (
regardless of the value of the load), with the amplifier's volume set at maximum, will be: the square root of [40 watts x 8 ohms] = 17.9 volts. With a 19.2:1 step up transformer (i.e. a 230:12 turns ratio) the peak output voltage across the secondary of the transformer (i.e. grid-to-grid of the modulator tubes), with the amplifier's volume set at maximum, will be 17.9V x 19.2 = 343 volts.
Stu